In the state of the art, tuning mechanisms and tuning circuits have employed various devices to provide the voltage-variable capacitance function needed for effectively tuning RF circuits.
As is well known to those skilled in the art, varactors are variable capacitance devices in which the capacitance is dependent upon a voltage applied thereto. As such, varactors have been commonly employed in RF tuning applications because the capacitance variations of the varactor caused by an applied voltage has corresponding effects on frequency tuning. In order to have a maximum effect on the tunability of a circuit, the varactor must be placed in a position of maximum standing wave voltage in the tuning circuit because the mount of tuning is dependent on the voltage-controlled capacitance variations resulting from changes in the semiconductor depletion region capacitance in the varactor. Consequently, varactors are typically characterized in terms of the range of capacitance variations and the breakdown voltage.
Semiconductor-based varactors have been specifically used in a various number of applications through the years, but nevertheless have numerous disadvantages. Most notably, the inherent properties of semiconductor materials cause these semiconductor varactors to be susceptible to overheating and burnout if forward biased or reverse biased with an excessive applied voltage. Specifically, semiconductor p-n junction devices have a depletion region that is subjected to high electric field stress, and as a result, the semiconductor devices tend to break down as the applied voltage is varied. Furthermore, the breakdown voltage of semiconductor devices is not easily scalable because the depletion region is fixed and the doping of the p-n junction must be altered to change the breakdown voltage characteristics. Moreover, semiconductor p-n junction devices typically have asymmetrical voltage characteristics as a result of current flow that is governed by the density and movement of holes and electrons. Furthermore, semiconductor materials typically have dielectric constants in the range of 10 to 15, and consequently, the capacitance of semiconductor-based varactors is limited by these lower range dielectric constants.
Even though thin film semiconductor varactors constructed from silicon compositions offer relatively high switching speeds and provide relatively high capacitive switching ratios (i.e., switching between the device's maximum and minimum capacitances), some applications require higher capacitances to provide a maximum effect on tunability.
To address the disadvantages of the semiconductor prior art devices, ferroelectrics have been increasingly used for various applications. The most notable applications include non-volatile memories, pyroelectric type infrared sensors, and to a lesser extent, RF applications. As is well-known in the art, some of the more desirable properties of the ferroelectric materials include the increased power handling capacity, low loss, large permittivity, as well as higher tolerance to burnout.
Ferroelectric varactors based on bulk cut material also exist in the field of art. However, the thickness of these devices typically limit the total capacitive effect. To address these capacitance limitations, thin film ferroelectrics are becoming more common, as evidenced by recent applications in the state of the an. The ferroelectrics predominantly used in thin film capacitance applications include dielectric materials such as barium titanate, lead zirconate titanate (PZT), and strontium titanate. The dielectric characteristics of these and other ferroelectric materials known in the state of the art offer significant advantages to overcome the limitations of semiconductor and bulk cut ferroelectric devices. However, the performance of devices using these thin film ferroelectric materials is dependent on numerous factors such as: the inherent properties of the ferroelectric compositions; the interaction between the thin film ferroelectric and the other layers in the device (e.g. reactivity between the thin film and the substrate or electrodes); the structure of the thin film device; as well as the thin film deposition techniques.
Several problems have persisted in the thin film prior art. For instance, the thickness of the thin film has been reduced in some devices to achieve higher capacitance; however, the resulting thin film is too thin and thus has poor film quality which negatively affects the performance of the device. Another drawback in the prior an is that a large leakage current may exist as a result of the close proximity of the dielectric thin film to the electrodes of a device. To address these limitations, some devices in the prior art have substituted for the conventional electrode material with a dielectric layer having added impurities to provide the electrical characteristics of an electrode, whereas some have used a thin film metal alloy to provide this needed functionality. However, these devices typically sacrifice some of the capacitive performance benefits that otherwise would result with the use of a more conductive material in combination with the ferroelectric thin film.
Conventional ferroelectric devices typically have another drawback with respect to the mismatched crystal structure of the various layers, specifically with regard to mismatched lattice constants. While some advances have been made to produce a structurally matched ferroelectric device, these advances have not produced a ferroelectric with an elemental composition that is ideally suited for use in RF tuning applications. In general, thin film ferroelectric devices in the state of the art are typically suited for particular applications, and a thin film ferroelectric device with optimal characteristics for RF tuning applications has not yet been provided.